Mt Pinatubo's brimming lake threatens thousands

 作者:尉迟嫣钲     |      日期:2019-03-06 10:13:19
By Joanna Marchant Pinatubo, the Philippine volcano that erupted 10 years ago killing hundreds of people, may be about to wreak havoc of a different kind among the people living in its shadow. What’s more, the authorities are doing little about it. Since the last eruption, the volcano’s crater has been filling with rainwater and the water is now only a few metres from the rim. If heavy rains continue the rim could be breached, and a wave of water and volcanic debris sweep through towns and villages below, say geologists. Kelvin Rodolfo, an expert on Mount Pinatubo from the University of Illinois in Chicago, and Rosalito Alonso, head of geology research with the British aid charity Oxfam, have been trying in vain to persuade the Philippine authorities to prepare for such a flood. “We can’t ignore the fact that we have several hundred million cubic metres of water sitting up there, and a town down here,” says Alonso. “Somebody has got to say something,” says Rodolfo. Pinatubo’s 1991 eruption was one of the most violent of the 20th century. Hundreds of thousands had to flee their homes as huge swathes of the Philippines were turned into a grey desert. Since then, many local people have returned to their homes. Few, however, know about the crater lake that now holds around 250 million cubic metres of water. With a local annual rainfall of more than 3.5 metres and the funnel effect of the crater, the water level has been rising by up to 10 metres each year. This year has been particularly wet, and since the rainy season began in June the water level has been rising by up to a metre each week. The last time Rodolfo and Alonso flew over the lake in a military helicopter, they estimated that the water was only four metres from the lowest point of the crater wall, a V-shaped cleft called the Maraunot notch. The geologists predict that when the water reaches the notch it will spill over the edge into the Maraunot river. Once the water starts to spill, they fear the crater wall could give way. Chris Newhall of the University of Washington in Seattle led a field trip to Pinatubo in May. His group found that at least the top 10 metres of crater wall is made of volcanic “breccia” – angular fragments of sand and rock deposited by the last eruption. If water starts running over the top, Newhall says, the breccia will crumble. “When the water reaches the top it could begin eroding downward until it hits rock that is solid,” he says. Rodolfo and Alonso estimate that if the notch eroded down 10 metres, 30 million cubic metres of water would spill out. The water would probably mix with loose volcanic sediment on the way, hugely increasing its volume and creating a cement-like mixture knows as a “lahar”. Rodolfo has seen fast-flowing lahars in action before. “It is a horrendous sight – horrifying and exceedingly beautiful.” After hearing about Newhall’s work, and realising that nothing was being done, Lan Mercado, Oxfam’s representative in the Philippines, asked Rodolfo and Alonso to identify the risks to local people. There are two hamlets built on the riverbanks and at the bottom of the slopes is the town of Botolan, with 48,000 inhabitants. At the rate the water is rising, the geologists estimate that the crater wall could give way in less than a month, letting a lahar plough straight through Botolan. “The town would have less than two hours’ warning. There is no plan. We want people to know about it and be prepared,” says Alonso. National authorities have been slow to respond to Oxfam’s warnings. Last month, Rey Punongbayan, director of the Philippines Volcanology and Seismology Agency commissioned a report into the lake by Masao Chida, an engineer from the Japanese consultancy firm Nippon Koei. Chida visited the crater by helicopter and concluded that there was little likelihood of a serious breach. “We will just watch what happens this rainy season,” says Florente Soriquez of the Department of Public Works and Highways. Engineering solutions to the threat include reinforcing the notch, or boring a tunnel into the crater to drain the lake. But the government says it is too late for such action now the rainy season has started. Rodolfo and Alonso believe this complacency could be disastrous and are now working with Oxfam to reach local agencies and people directly. They are urging people to find safe buildings, arrange shelter and supplies, and monitor the rivers for rising water levels. Oxfam’s actions have alarmed the government. Oxfam “should not announce their findings in public”, says Punongbayan. “It may make people nervous.” But Mercado counters: